Fishman Loudbox 100 mute circuit

I have a Loudbox that came in with no channel 2 sound, and no phantom power. It's an interesting design. Channel 2 uses a tip/ring/sleeve jack. If you use a stereo plug, then the tip signal goes to channel 2, and the ring signal goes to channel one for utilizing a two pickup system, or pickup/mic. If I push a mono plug in halfway, the signal goes to Channel 1. So it is the tip signal of channel 2 that we are losing.

Fed with a signal generator I get signal up to U3C pin 8 on page 3 upper right. Curiously, I see a lesser signal on R67 at the front door of the CH2 Mute circuit, which I wonder about since they are indicated as the same point electrically. Anyhow, I suspect this mute circuit, or even one of the others that I see downstream. There seems to be no shortage of them.

Mute circuits tend to vex me. And the way this unit is constructed makes access very difficult the way the Ch1 and Ch2 preamp boards are sandwiched and cabled together. And surface mount components as well. Probing is difficult to say the least, but I did manage to trace the signal to U3C by unplugging one cable connecting the two preamp boards. This interrupts the send/return circuits, so the power amp receives no signal, but allows for some tracing at least.

I could use some guidance on what I might do to see if this or other mute circuits are on when they shouldn't be. I can tell you the front mounted master mute is working. It seems there is a separate CH2 mute, but I don't see why or how it is triggered.

There is just the one mute. It mutes both channels. On sheet 5 top left there is a handy mute signal truth table. The tip path mute has a pass FET Q5 and a shunt FET Q6 ( for extra attenuation). To pass the signal Q5 will be low resistance (gate around 0v) and Q6 high resistance (gate negative). For sure R67 should be the same signal as U3 pin so did you check both ends of R67?

That resolved check for signal on the drain of Q5 ( not muted). If the gate signals are OK then one or both of the FETS must be bad. You can use an ohmmeter between source and drain to check if a particular FET is high or low resistance.

Thank you nickB. My mistake, I was looking at the wrong IC. I don't have but a very small signal at U3C pin 8, but I suspect that is because it is being shunted to ground. And Pin 8 and R67 are indeed the same.

I'm a bit confused, I have about 35 ohms source to drain on both Q5 and Q6, powered up. But I also have -14v on R68, and =14v on R66. According to the handy mute signal truth table, doesn't that put it in mute mode?

OK. For sanity's just check that MUTE and MUTE2 are changing with the mute switch, MUTE2 should always be the opposite of MUTE. Set the mute switch off (unmuted) so that MUTE is low and MUTE2 is high. Check these signals make it to he gates of Q5 and Q6 noting that the didoes will prevent the gate from being positive, if not fix it. The check Q5 is low resistance and Q6 should be high.

Given the measurement from earlier it sounds like Q5 is bad but I'd like to do these extra checks with it in the unmuted state.

Carumba! I erred again. I had mistated the voltages on mute and mute2. What I actually have is +14v on R68 <Mute>, and -14v on R66<Mute2> with mute switch in OFF position. These voltages are opposite when mute switch is in ON position.

With Mute switch back to OFF position, I see 0v at gate of Q5, and -13.8v with mute switch in ON position.

With mute switch in OFF position, I see -13.8v at gate of Q6, and 0v with mute switch in ON position.

With mute switch in OFF position Q5 drain to source is 36 ohms, with mute switch in ON position it is 32 ohms.

With mute switch in OFF position, Q6 drain to source is 42 ohms, with mute switch in ON position it is 37 ohms.

The voltages at the gates seem to make sense, but the slight change in resistance between the drains and sources, not so much?

edit: I just pulled Channel 1 board and did the same tests, and got the same results. Is it proper to measure source/drain resistance while powered up, or am I getting skewed readings?

The problem with the measurement is that U3 is DC coupled to the FETs so it low output impedance is screwing with them. If with muting off there is no signal on the drain of Q5 then just replace Q5 and Q6. They are cheap and it saves time to do them both.

Thanks again nickb. I will take that advice abd replace Q5 and 6, but can you explain the logic here regarding Q5? If no signal on the drain (pin 1), would that mean that it is somehow pulling the signal down, and if so, how?

Perhaps I should remove Q5 first to see what the signal looks like on pin 8 of UC3?

Also I find the SST4392 is obsolete. Is there a sub you might recommend? This perhaps?

In the OP you said "Fed with a signal generator I get signal up to U3C pin 8 on page 3 upper right." Is that no longer true?

If true does the mute effect it? If it does then the FETS are shorting the opamp output to ground as one of the FETS is bad. If it's never there than you need to fix that and it could be both FETS bad or something else. Check the input to U3 on R47.

Think of the FETS as switches that are either a few ohms or a megohm or so controlled by the gate. Normally when muted Q5 is open and Q6 closed and then unmuted Q5 is cloded and Q6 is open. If Q6 is stuck on then when unmuted Q5 will also be closed and the signal will have a low resistance path to ground. If Q5 were stuck on then when muted Q6 will be closed and shorted to ground.

"In the OP you said "Fed with a signal generator I get signal up to U3C pin 8 on page 3 upper right." Is that no longer true? "

Yes, I did say say that initially, and then corrected my mistake later. I was looking at the wrong IC initially, as the layout was not what I would have expected. The actual truth is I have no signal at the proper U3C pin 8, and thus no signal anywhere at Q5.

I believe I understand how the Jfets should be acting in theory.

Forget what I misinformed previously, if I have no signal at the proper U3C pin 8, as well as Q5 pin 1, with mute switch not effecting this, do you still think this points to either Q5 or 6, or should I be looking at a possible U3C failure?

I am really trying to understand why there is no signal coming off U3C pin 8. I have no problem replacing several components, but would like to order everything I might need in one time.

AHA! Upon making these measurements (double checking to make sure I am on the correct ICs) I find a nice strong signal on the + side of C9 following U1 Pin 7, and a weak signal on the other side. I removed it, and sure enough it is bad. I had an extra one from a Loudbox repair from 4 years ago, I put it in, and voila! So it wasn't a mute issue after all. I am humbled by my error in misinforming my results by way of measuring at the wrong IC, and the extra effort that caused. Thank you nickb for staying with me on this, you are a talented elctron whisperer!

However, now that I have it working, I notice that channel 2 sounds good, but it is not as loud as channel 1. I'm not sure if this is by nature, as it does differ from channel 1 in that it has a mic XLR input. It would not seem so from the block diagram, as in I don't see any extra gain stage. So now I wonder is this job done, or perhaps do I have another bad cap impeding things?

I split my test signal and fed into both FX return jacks, channel 1 is louder. I activated the mute switch, and had a look at the FX return circuit. Good strong signals on U3 pin 7 on both boards, and good strong signal on U3 pin 8 on channel 1, but a weaker signal by about 1/3 on pin 8 channel 2. Again I briefly shorted C32 following U3 pin 7, and once again the volume increased. Sadly, I have used my last one, and must order more. I think these caps are definetly a common problem with these Loudboxes this is the 4th 10uF/16v one I have replaced.

Well, to sum this job up, I found yet a third bad 10uF/16v cap. So C9 was halting the signal in Channel 2, replaced it, but ch 2 wasn't as loud as ch 1. Found a bad C32, this increased the volume only slightly. Found a third bad C7, between the first and second stages of U1. Now Ch 2 barks every bit as loud as Ch 1.

So, this is the second time I have replaced multiple 10uF/16v SMD capacitors in a Loudbox. I bought a strip of 10 to keep on hand. Seems these are prone to failure. Seems to me the signal must have been degraded before it finally cut out. Made it a bit tricky at the beginning trying to understand what I was seeing. But it's all good now.

As always, thanks for your help guys! Especially when I was misleading.